This invention relates to a discharge lamp lighting apparatus used for an automobile or as a light source of a projection display.
FIG. 18 is a circuit configuration diagram showing a conventional discharge lamp lighting apparatus disclosed in the Unexamined Japanese Patent Application Publication No. Hei 12-82592. In the drawing, numeral 1 is a direct current power source such as a battery, and numeral 2 is a DC-DC converter for regulating and outputting electric power supplied from the direct current power source 1 and in the DC-DC converter 2, numeral 2a is a transformer and numeral 2b is an FET (Field Effect Transistor) and numeral 2c is a diode. Numeral 3 is a ground, and numeral 4 is a shunt resistor for detection of a discharge lamp current IL, and numeral 50 is a full bridge circuit (hereinafter called “H bridge”) including of FETs 50a to 50d connected in an H shape and converting direct current electric power regulated by the DC-DC converter 2 into alternating current electric power, and numeral 6 is a discharge lamp driven by the alternating current electric power converted by the H bridge 50.
Also, numeral 7 is an interface (thereinafter called “I/F”) which receives a discharge lamp voltage VL from the side of a cathode of output of the DC-DC converter 2 and also receives the discharge lamp current IL from the side of the H bridge 50 of the shunt resistor 4, and numeral 8 is a microcomputer for controlling the FET 2b of the DC-DC converter 2 so that electric power supplied to the discharge lamp 6 based on the discharge lamp voltage VL, the discharge lamp current IL and a preset circuit impedance fixed value sequentially detected through the I/F 7 is brought to a predetermined value.
Next, operations will be described. In the case of starting to light the discharge lamp 6, the electric power supplied from the direct current power source 1 is regulated and outputted by the DC-DC converter 2 and further the DC electric power is converted into AC electric power by the H bridge 50 to drive the discharge lamp 6. Here, the discharge lamp voltage VL detected from the side of the cathode of the output of the DC-DC converter 2 is increased to −400 V as shown in FIG. 19 and is further increased to about 20 kV in the peak and then the discharge lamp 6 lights and thereafter reaches a stable state of lighting at −90 V. Such control is performed by controlling the FET 2b of the DC-DC converter 2 so that the electric power supplied to the discharge lamp 6 based on the discharge lamp voltage VL and the discharge lamp current IL sequentially detected through the I/F 7 is brought to the predetermined value by means of the microcomputer 8.
After discharge lamp 6 lights, an AC voltage is applied to the discharge lamp 6 by repeating the switch state with the FETs 50a and 50d of the H bridge 50 turned on and the FETs 50b and 50c turned off and the switch state with the FETs 50a and 50d turned off and the FETs 50b and 50c turned on.
By the way, it is desirable that the discharge lamp electric power supplied to the discharge lamp 6 in the stable state of lighting be 34 W. However, in case of merely controlling the electric power supplied to the discharge lamp 6 based on the discharge lamp voltage VL and the discharge lamp current IL at 34 W by the microcomputer 8, there is a loss due to a voltage drop due to the ON resistance of the FETs 50a to 50d of the H bridge 50, so that the electric power supplied to the discharge lamp 6 actually becomes lower than 34 W. Thus, the circuit impedance fixed value is set previously in expectation of the drop due to the ON resistance of the FETs 50a to 50d of the H bridge 50 and on the basis of the discharge lamp voltage VL, the discharge lamp current IL and the preset circuit impedance fixed value by the microcomputer 8, and controlled so that the electric power supplied to the discharge lamp 6 is 34 W even when there is the power loss due to the drop by ON resistance of the FETs 50a to 50d of the H bridge 50.
The conventional discharge lamp lighting apparatus is configured as described above and a high voltage of a maximum of 400 V is applied to the H bridge 50, so that the FETs of the H bridge 50 need to have high withstand voltages capable of withstanding 400 V. Such FETs having high withstand voltages are high in unit price and also four FETs with such a high unit price are used in the conventional configuration. Therefore, a configuration of an inverter circuit with the H bridge as described has an adverse effect in achieving miniaturization and cost reduction. A reduction in the number of FET elements of the H bridge 50 and a decrease in the voltage applied to the H bridge are problems of the discharge lamp lighting apparatus to be solved.